Rechargeable batteries are widely used in portable or mobile applications, devices and/or appliances and back-up power supplies. Cellular and cordless telephones, remote repeaters, remote control units, remote sensors, portable lighting devices, portable radios, portable drills, digital cameras are common examples of such applications, devices and appliances. Rechargeable batteries are preferred over disposable batteries because they have a longer operating life, are more environmental friendly and offer longer-term cost savings. For remote applications, rechargeable batteries are probably the only practical choice.
In general, rechargeable batteries may comprise a single battery cell or a plurality of battery cells connected in series. The voltage of each battery cell is typically between 1-2 volts and more commonly in the region of 1.2 to 1.5 volts. For example, a typical dry rechargeable Nickel Metal Hydride (NiMH) battery cell has a rated voltage of 1.2 volt.
As a result of improvements in battery technologies and the increasing demand for batteries with a higher power density to feed high drain rate devices such as digital cameras, standard-sized batteries with enhanced capacities are available. For example, dry rechargeable AA batteries with a capacity of over 2000 mAH and above are now commonly available and further enhancement in battery power density can be expected. The GP® 2100 series NiMH batteries from the Gold Peak® Group are an example of high power density rechargeable batteries for general application.
Rechargeable batteries require repeated charging to provide renewed energy for repeated discharging and battery chargers are provided for such purposes. A typical battery charger comprises a charging power source and battery charging circuitry. The charging power source usually comprises either a constant current source or a constant voltage source. Battery chargers commonly utilise high frequency pulsed charging current with a relatively high current rate for fast charging. For example, a 4C current source is used for a 15-minute fast battery charger. The constant voltage or constant current source usually comprises a switch mode power supply (“SMPS”) which converts the 50 or 60 Hz AC mains power into a charging power of a high frequency, for example, by chopping at between 10 kHz to 100 kHz, although a chopping frequency of above 20 kHz is preferred to mitigate audible noise.
In many applications using batteries, the power supply of a device is formed by a connection of a plurality of batteries, for example standard sized batteries such as A, AA, AAA, C, D or 9-volt batteries, and the operating voltage is usually significantly less than the AC mains supply voltage. For example, many portable devices are powered by 4 to 6 standard sized batteries and the maximum voltage is usually less than 9 volts. Typically, the charging power source is usually conveniently obtained from the AC mains. To provide a suitable voltage for battery charging as well as safety isolation to protect users from electric shock and to comply with various safety regulations and standards, isolation transformers are usually included in a battery charger and concealed within an insulated housing. Of course, the reference to standard sized batteries is for example only and batteries of new sizes and/or types will become “standard-sized batteries” as demands justify. For example, lithium-ion batteries may be available in standard sizes.
The widespread use of rechargeable batteries, especially in consumer applications, also sees an increasing demand on fast battery chargers. The term “fast battery chargers” is commonly understood by persons skilled in the art as referring to battery chargers which are capable of charging an empty battery to its fully charged state within an hour or less. A fast battery charger which are designed to fully charge a battery to its fully charged state in one hour is commonly known as a “1C” charger and such a battery charger is equipped with a “1C” current source or a current source with a “1C” rating. For example, for a rechargeable battery of 2,000 mAH capacity, the 1C charging current rate is 2A and the 2C charging current rate is 4A and so forth. In the exemplary case of a 15-minute fast battery charger designed for AA-sized batteries of 2,000 mAH capacity, a charging current of 8 Amperes will be required for each battery charging section. If a parallel-type battery charger topology is adapted so that a plurality of battery charging sections are connected in parallel to a charging power source, the total charging current will be multiplied by the number of parallel charging sections. Thus, for a battery charger with, say, four charging sections, a charging current source of 32 Amperes rating is required and so forth. This phenomenon would mean that the parallel charger topology is less attractive for fast battery chargers, especially when the charging speeds, battery capacities and charging section increase further.
Hence, it will be apparent that fast battery chargers, especially those for charging small voltage re-chargeable batteries of voltage rating of about 1.2-2V, are preferably configured as the serial-type charger with a plurality of battery charging sections so that the batteries can be charged in series. Otherwise, a power supply with a very large current supply rating will be required and this may be very bulky and costly.
On the other hand, a conventional serial battery charger design implies that the same charging current will flow through each of the connected battery charging sections. This may create difficulty in many circumstances. For example, it may be necessary to remove or isolate a battery from a charging section when charging is completed or because the battery is defective. This may happen while other batteries in the charger are still under charging conditions. When a battery is removed from a charging section, charging will be interrupted due to the series connection of the battery charging sections unless alternative paths are provided. Similar problems also arise if rechargeable batteries of different capacities are charged together or good batteries are mixed with bad or wrong ones. For example, when a battery of a smaller capacity has been fully charged, there is a good chance that a battery of a larger capacity still requires charging. For ordinary serial chargers with less sophisticated monitoring and control circuits, the batteries will be indiscriminately charged. Consequently, overheating, battery damage or even explosion may result. On the other hand, for serial battery chargers with more sophisticated charging conditions monitoring and control circuits, the battery charger may be shut down once any one of the batteries has been fully charged. This is unsatisfactory as the remaining batteries may still require further charging. In many battery chargers, it is known that, when power supply to the battery charger is turned off, a reverse leakage current may flow from the battery to the charger or the peripheral circuitry. Such a reverse leakage current may cause reverse charging of individual batteries by other batteries that are connected in a serial charger. This is clearly undesirable since batteries may be drained and the charger may be damaged.
Hence, it will be apparent that several requirements need to be overcome simultaneously by a serial battery charger if shortcomings of conventional serial battery chargers are to be alleviated. Firstly, in order to prevent adverse reverse current leakage or current discharge from a battery, a blocking device having a high reverse impedance needs to be inserted in series with a battery in a battery charging section. Secondly, that serial blocking device must have a very low-impedance for a forward current which flows into the battery for battery charging. On the other hand, if the blocking device has a low forward impedance when the bypassing switch has been activated (which usually occurs when there is still power supply to the battery charging terminals), that low-impedance blocking device will compete with the bypassing switch for the supplied current and, as a result, adverse charging current will keep flowing into the batteries. In addition, that blocking device must have a high-impedance when the bypassing switch has been activated, otherwise, a large and un-desirable current will flow in a current loop comprising the battery, the blocking device and the bypassing switch. Serial chargers meeting the above requirements have been disclosed in the parent applications, namely, U.S. Pat. No. 6,580,249 (under Re-issue process) and U.S. patent application Ser. No. 10/383,613 published as US 2003/0160593, which are incorporated herein fully in their entirety by reference.
As fast battery chargers are expected to become faster and faster, for example, a fast charger with a charging current source of 4C is already available, a complete battery charging cycle time is expected to reduce further and a full charging cycle of less than 15 minutes can be expected in the short near future. To cope with this trend of development, fast and more responsive battery charging, monitoring and control circuitry is needed to timely terminate battery charging to alleviate the risks of adverse consequence due to overcharging by a large current source. Accordingly, fast battery chargers with improved battery charging control and monitoring circuitry and methods, which are faster and more responsive, are desirable and beneficial.